The response from the Google Calendar team read as follows:
Thank you for your patience. We’ve made some changes recently that may
have resolved the problem. Please let us know if this error persists.
The Google Team
I went into my settings in Google Calendar and added my number in the space provided in the international format (+27831112222 – not my number) and I received a confirmation code by text message within seconds. I have an event in my Google Calendar schedule later today so I’ll see then if the notifications arrive and if they arrive on time.
If you are wondering why this is an issue, I believe it is because event notifications in Google Calendar seems to be handled system wide and not on an individual event basis so this means you set notification details which do not carry across if you subsequently subscribe to a calendar in iCal or Outlook. I discovered this when I moved some of my calendars to Google Calendar and then subscribed to them in iCal. I quickly discovered that there were no event notifications imported into iCal along with my calendar entries and that I had to rely on notifications sent my Google Calendar. Notifications by sms/text message are a solution that, while not ideal, may just work for the time being.